Inductive plasma reactors, which typically consist of reactor chambers and inductively coupled plasma (ICP) sources, are commonly recognized as advanced, convenient, and cost-effective devices for plasma processing of parts and materials at various stages of large-scale manufacturing, e.g., of semiconductor chips and large panel displays. Such sources are also used for activating gases needed for cleaning plasma-processing chambers and for incineration (abatement) of harmful plasma processing gases, [see: M. A. Lieberman and A. J. Lichtenberg, “Principles of Plasma Discharges and Materials Processing”, John Wiley & Sons, Inc, New York, 1994]. Application of inductive discharges has an advantage of achieving high-density plasma in a wide range of gas pressures with efficient energy transfer to the plasma electrons rather than to the plasma ions as is typical of capacitively coupled RF discharges.
A typical ICP reactor for large area uniform plasma processing of 300 mm wafers and flat panel displays comprises a cylindrical metal chamber filled with working gas and having a dielectric (quartz or ceramic) flat, dome-shape, or cylindrical window separating an inductor coil plasma source from the plasma in the working chamber where an object, e.g., a semiconductor wafer, is located for treatment. The operation of this ICP (as of any inductive discharge) is based on the principle of electromagnetic induction. The RF current driven in the inductor coil induces an electromagnetic RF field and RF plasma current in the activated gas of the working chamber, thus maintaining the plasma discharge inside the chamber. As any inductive RF discharge, an ICP source can be considered as an electrical transformer where the inductor coil connected to an RF source is an actual primary winding and the plasma is a single closed turn of a virtual secondary winding.
An ICP antenna loaded with plasma has mainly inductive impedance (reactance) that has to be compensated with matching-tuning network (matcher) for impedance matching conditions required for efficient transfer of RF power from an RF generator to the plasma-excitation antenna of the plasma source. The matcher is connected to the coil with a conductor conduit bearing a large resonant RF current, thus distorting axial symmetry of electromagnetic field distribution created by the ICP antenna. Asymmetry of the RF field results in plasma azimuthal asymmetry and non-uniformity of treatment. When the length of the conductor becomes comparable to the coil length, the asymmetry effect caused by the conduit becomes large, especially for large ICP reactors used for treating 300 mm wafers, the plasma-excitation coils of which have a low number of turns, and, hence, low inductance. On the other hand, matching of the coil having low inductance with a standard resonance matching network becomes energy inefficient due to extra-large RF current and thus high power losses in the matcher network and in the coil itself.
A common problem that occurs in industrial ICP reactors for plasma processing results from a high RF voltage (a few kV) between the terminals of the inductor coil (antenna).
High antenna RF voltage requires special means for adequate electrical insulation and leads to considerable capacitive coupling between the coil and plasma. The non-linear electromagnetic interaction between the field of the RF coil and the plasma sheath on the inner surface of the reactor window creates a high negative DC potential in the interaction area. This negative potential accelerates the plasma ions towards the window causing erosion and sputtering of the latter whereby plasma is contaminated.
Another problem inherent in plasma processing ICP sources is a transmission line effect along the coil conductor. This effect occurs due to capacitive coupling of the induction coil to plasma and/or to grounded parts of the plasma reactor, resulting in the coil current non-uniformity along the coil wire, thus leading to the plasma azimuthal non-uniformity. The transmission line effect is an increasing problem for large RF plasma reactors when the coil wire length becomes comparable to the wavelength of the working frequency.
To reduce the capacitive coupling between the coil antenna and plasma, Faraday shields of different types are usually placed between the coil and the window. However, the shield introduces a significant additional RF power loss and significantly increases the transmission line effect, thus, deteriorating plasma uniformity. Also, the presence of the Faraday shield makes it difficult to initiate discharge in the processing reactor. That is why Faraday shields have not found wide application in commercial plasma reactors.
Another way to reduce capacitive coupling is to balance the induction antenna by feeding it with a symmetric (push-pull) RF power source [U.S. Pat. No. 5,965,034] or by connecting the second coil end to ground through a balancing capacitor [U.S. Pat. No. 6,516,742 B1]. In both cases, the coil ends acquire nearly equal but opposite phase RF potential references to ground, thus forming a virtual ground point in the middle of the coil and reducing about twice the maximal coil potential reference to ground on the both coil ends. This way of the coil RF potential reduction is widely used in commercial ICP reactors. It provides a certain benefit in capacitive coupling reduction, but still far not enough for essential reduction of the capacitive coupling and of the transmission line effect.
Induction coil antennas immersed into plasma are used in many ICP sources for ion accelerators. Furthermore, such immersed antenna coils find application in very large plasma sources for processing large plasma displays [Deguchi et al, Jpn. J. Appl. Phys. 45, 8042 (2007)]. The use of immersed internal antennas results in enhancement of inductive coupling that increases the ICP energetic efficiency. A drawback of an internal coil antenna is an increased capacitive coupling to the plasma sheath surrounding the antenna wire. The rectification of RF voltage in the sheath causes ion bombardment of the coil leading to its sputtering and plasma contamination.
The recessed antenna surrounded by cup-shaped reentrant cavity to reduce the plasma sheath interaction with the coil is disclosed in U.S. Pat. No. 5,309,063 issued to Singh. The antenna coil in this patent has a flat portion (on the cavity bottom) and a cylindrical portion on the cavity side. The cavity diameter is close to the diameter of the processing wafer. Therefore, the cylindrical portion of the antenna coil enhances peripheral plasma, thus improving plasma uniformity over the wafer processing area.
A plurality of immersed coil antennas of different configurations in recessed reentrant cavities is disclosed, e.g., in U.S. Pat. No. 6,259,209. Distribution of multiple immersed antennas over the plasma processing area allows for generation of uniform plasma over a large processing area.
Another common problem inherent in the ICP reactors of the types described above is inefficient removal of heat generated by the coils activated for excitation of plasma. Attempts have been made to solve the heat-removal problem in RF plasma reactors. For example, U.S. Pat. No. 7,137,444 issued to V. Faybishenko, et al. discloses a heat-transfer interface device for use in a range of up 320° C. working temperatures for removal of heat from RF coils used in an inductively coupled plasma reactor. The device comprises an elastomeric material filled with an electrically nonconductive and thermally conductive filler material. The elastomeric material may have recesses on the surface or the surface may be curved, e.g., on the side facing the source of heat for forming a space between the surface of the device and the mating surface of the source of heat. The elastomeric material is clamped between the heat source and heat receiver in a compressed state so that when it is expanded under the effect of an increased temperature, the material is redistributed and the recesses are flattened. The elastomeric material comprises perfluoroelastomer polymer, and the filler can be selected from boron nitride, aluminum nitride, beryllium oxide, etc. If necessary, a combined mixing-assisting and compression-set-reducing agent in the form of perfluoropolyether can be added.
U.S. Pat. No. 6,178,920 issued to Ye, et al. discloses an internal inductive antenna capable of generating plasma. In the preferred embodiments, the internal antenna of the present invention is constructed to prevent sputtering of the antenna. In one embodiment, for example, a bell shaped glass jacket with a hollow interior prevents sputtering of the conductive coil, while allowing rotation, shielding, and temperature regulation of the antenna. A main disadvantage of the antenna arrangement of this patent is that the antennas generate plasma in a certain plane which defines the plasma volume. This is associated, probably, with difficulties in removal of heat from the coils. Furthermore, such an arrangement of the coils makes it difficult to effectively transfer the RF energy into the plasma.